This invention relates to printed circuit boards and, more particularly, to a machine and process for reworking defective printed circuit boards whose vias and plated through holes (PTHs) have become plugged by excessive solder.
In the past, defective printed circuit boards have been recycled by using a hot air solder leveling tool to clear solder-plugged vias and plated through holes (PTHs). Such a process, however, required prolonged heating of the entire printed circuit board. The prolonged heating often damaged the circuit board, so the proposed process could never be implemented reliably. present invention is a new machine and method for clearing the plugged vias and PTHs of printed circuit boards. The new machine and reworking process utilize localized slow heating of the PTHs that minimizes thermal shock. In addition, the invention does not physically contact the PTHs and surrounding land, thus avoiding or minimizing damage to the metal plating during the reworking process. A plugged hole on the defective panel is heated from below by convection; a subsequent air blast clears the molten solder therefrom.
In U.S. Pat. No. 3,690,539, issued to Geiger on Sep. 12, 1972 for HEAT AND AIR ACTION APPARATUS FOR ELECTRONIC CIRCUITRY REPAIRS, a heat and air suction apparatus is described that can be used for repair of electronic circuitry. The device is capable of melting the solder. An air conduit links a hollow heater tip to an air connection. The conduit has a solid matter separator.
In U.S. Pat. No. 3,987,854, issued to Callihan et al on Oct. 26, 1976 for GRAVEL PACKING APPARATUS AND METHOD, a solder removal device is shown. The device has a handle with a venturi mechanism for creating a vacuum. The vacuum is applied to a solder collection chamber which, in turn, is heated by hot air. A jet of hot air is directed from a tube that melts and blows the solder away.
In U.S. Pat. No. 4,419,566, issued to Fortune on Dec. 6, 1983 for HOT AIR SOLDERING AND RESOLDERING SYSTEM, means are illustrated for issuing hot air from a jet tip having an axial bore. The air is heated to liquefy a small amount of solder, thereby allowing soldering and desoldering to occur.
In U.S. Pat. No. 5,031,820, issued on Jul. 16, 1991 to Jacks et al for PCB REWORK STATION, a rework station for soldering/desoldering of integrated circuits on a printed circuit board is illustrated. A base supported gantry directs hot gas delivery and a vacuum over a printed circuit board.
In U.S. Pat. No. 5,758,815, issued to French et al on Jun. 12, 1998 for SOLDER APPARATUS AND METHOD, an apparatus and method is described for soldering pin-in-hole electronic circuit components on a printed circuit board. Hot gases are directed through a matrix of holes and aligned vias, in order to reflow solder in the vias. The device provides localized heating to the solder joint, so that the board is not burned and other components are not reflowed.
In accordance with the present invention, there is provided a machine and method for repairing printed circuit boards whose vias and plated through holes (PTHs) have become plugged by solder. The machine comprises an adjustable, movable support for holding the printed circuit board that includes defective PTHs. The support positions the defective area of the printed circuit board under an air nozzle. A heating device positioned below the defective area heats the board to at least 420xc2x0 F., for approximately five minutes or less. The heat melts the solder in the PTHs; the air nozzle blows the melted solder into a collection pan disposed below the printed circuit board. A solenoid or stepper motor is controlled by a timed switch or computer. The computer actuates the solenoid to activate an air valve which supplies compressed air to the air nozzle after moving the heating device away from the defective area.
The printed circuit board is heated from the bottom side. A compressed air blast is applied to the board when the solder in the PTHs is in a molten state, as aforementioned. The compressed air is more effective than is vacuuming in displacing molten solder. The defective area is masked to prevent solder splash back and to localize the defective area. The heating device is adjustably positioned a given distance from the board to provide localized heating thereto. The heat is automatically withdrawn from the defective area, while cool blasting air is allowed to blow over the affected area and through the PTHs.
The process of this invention comprises the following steps:
a) taping the printed circuit board (PCB) with a high temperature tape to expose only the defective area containing a solder plugged PTH;
b) adjusting a movable support to fit the size of PCB that is to be reworked;
c) placing a heating device proximate the defective PTH area;
d) heating the PCB about the defective PTH area until solder in the PTH is substantially completely melted;
e) removing the heating device from the defective PTH area; and
f) applying air under pressure to the defective PTH area to carry away excessive molten solder from the PTH and to cool the PCB.
It is an object of this invention to provide an improved printed circuit board reworking apparatus and method.
It is another object of the invention to provide a reworking apparatus and method for removing solder from plugged plated through holes disposed upon printed circuit boards.